Image forming apparatuses, e.g., full-color multifunction peripherals and printers, take advantage of electrophotography form individual color images by using full-color toners respectively, e.g., a yellow component, a cyan component, and a magenta component, and superimpose toner images of individual colors. In general, the toner includes base particles, which contain a colorant and additives in a binder resin. The toner may be used alone as a one-component developer or be used as a two-component developer in which the toner is added to a carrier. Such toners are required to improve the hue, the chroma, and the lightness of the individual color components of the toner in a balanced manner in order that images formed by using the toners exhibit color reproducibility with maximum faithfulness to original images.
Properties of a color component may depend upon the colorant used in the toner. For example, properties such as the hue, the chroma, and the lightness of the color component may be affected by the colorant used in the toner. Incidentally, the colorant may exert an influence on the charging properties of the toners of individual colors on the basis of specific structures and differences in compatibility with binder resins. Consequently, at initial stage of printing, the additives and the carriers deliver their functions sufficiently and the toner performance required in individual processes, e.g., development, transfer, and fixing, can be satisfied. However, in some cases where printing is repeated for a long term, the charging properties of specific color components of the toner deteriorate, image fogging and toner scattering occur, and defective images result.
In this regard, a method for suppressing deterioration in charging property of the toner by using an external additive on surface of the base particles has been known. In many cases, titanium oxide is usually used as the external additive. Furthermore, for the purpose of stabilizing the charging performance in repetition of copying for a long term or in a specific environment of high temperature and high humidity, low temperature and low humidity, or the like, a developer has been proposed, in which a positively chargeable silica fine powder or negatively chargeable, fluororesin-containing fine particles are used as external additives.
However, past use of this method has resulted in cloudy images and a deterioration of the color developing property. Moreover, the yellow component exhibits particularly high lightness among full-color toners and, therefore, color development of a formed image is susceptible to the external additives.
Consequently, a yellow component including a monoazo pigment has been proposed as a component of a toner exhibiting excellent color reproducibility, having good coloring power, and good charging properties.
However, a yellow component including the monoazo pigment tends to lose charging characteristic. The amount of charge of the yellow component may decrease because of a structural feature of the monoazo pigment. In particular, the amount of charge decreases significantly after printing repeatedly for a long term, that is, after the toner undergoes a long-term agitation operation in a developing unit or a toner container. Hence, when a full-color image is formed, a difference in amount of charge occurs between the yellow component and the other color components of the toner (cyan component and magenta component). As a result, differences in developing properties occur between the individual color components of the toner and the color reproducibility of the image formed using the toner relative to the original image tends to deteriorate.